System for testing transistors



Sept. 18, 1962 s. BOSCIA ETAL 3,054,954

SYSTEM FOR TESTING TRANSISTORS Filed Oct. 14, 1958 MERCURY NOV 60 'b I27 2a 23 24 /y WA|IIIIIIIWQ E%' 7 /5 3 INVENTORS United States Patent3,054,954 SYSTEM FGR TESTING TRANSISTGRS Salvatore Boscia, East PlymouthValley, and Earl R.

Kreinberg, Graterford, Pa, assignors, by mesne assignments, to PhilcnCarper-ation, Philadelphia, Pa, a corporation of Delaware Filed Oct. 14,1958, Ser. No. 767,134 6 Claims. (Cl. 324-158) This invention relates tothe testing of transistors, and while the invention is applicable todiiierent modes of testing, it is particularly useful for determinationof the voltage level at which a phenomenon comparable to areing occurs,which would destroy the transistor if it were allowed to prevail evenfor a short time.

In some uses of transistors the destructive value of the collectorcurrent, commonly known as I is very important. For example, in class Aoperation of a transistor output stage of an auto radio receiver, it wasfound that some transistors would be burned out on strong signal due tothe fact that the collector current would reach the I value at which asolid state are occurs.

To avoid such destruction, where transistors are to be employed in anyusage which subjects them to possible burnout, it is necessary to testthe transistors beforehand to determined whether each transistor issuitable for the purpose to which it is to be applied. In the past,however, the test itself has caused destruction of some transistors. Insuch testing of a transistor, a test circuit is connected to the emitterand the collector, and increasing collector current is caused to flow inthe circuit while at the same time the trace of the collector voltagevs. collector current characteristic is observed on an oscilloscope. Inprior practice the operator observed the oscilloscope until it indicatedthe voltage valueat which the collector current increased abruptly. Thenthe operator quickly reduced the applied voltage to prevent damage tothe transistor. However, in many instances the transistor was damageddue to the fact that it was physically impossible for the operator toreduce the applied voltage quickly enough to prevent damage to thetransistor.

The persent invention solved the problem by providing a testing systemwherein the flow of collector current is automatically terminatedquickly enough to prevent damage to the transistor. Moreover, the systemprovided by this invention is applicable generally to, and facilitates,testing of transistors.

One object of the present invention therefore is to provide a system fortesting transistors in which the collector current is automaticallyterminated according to the purpose and conditions of the test in anyinstance.

Another object of the invention is to provide a system which isparticularly useful for the testing of the I characteristic and which iseffective in preventing damage to the tested transistors.

Other objects and features of the invention will be apparent from thedetailed description to follow.

In the accompanying drawings,

FIG. 1 is a diagrammatic illustration of a transistor testing systemaccording to the preferred embodiment of the invention;

FIG. 2 is an illustration of a typical collector voltage vs. collectorcurrent characteristic of a transistor, which will facilitate anunderstanding of various modes of test as hereinafter described; and

FIG. 3 is a diagramatic illustration of the same system as shown in FIG.1 with the addition of a pulsing arrangement for reducing the duty cycleof the test transistor.

Referring first to FIG. 1, a test transistor is shown at 10 having itsemitter and collector connected to a test circuit 11. An adjustableautotransformer 12 supplies an alternating voltage to said circuitthrough a transformer 13. Included in the circuit is a rectifier tube 14which may be cut off by the application of a negative voltage to itscontrol grid. By this arrangement, unidirectional collector current iscaused to flow through the test transistor, although it will be realizedthat with a halfwave rectifier as shown the current will be in the formof pulses. While full-wave rectification could be employed, the use ofhalf-wave rectification reduces the duty cycle of the transistor to 50and thus helps to reduce heating in the transistor.

Resistors 15 and 16 are serially included in the circuit 11, and anoscilloscope 17 is connected as shown so that the horizontal deflectionof the oscilloscope beam represents the collector voltage while thevertical deflection of the beam represents the collector current asdetermined by the voltage drop across resistor 16.

In accordance with the present invention, in order to effect automatictermination of the flow of the collector current as hereinbeforementioned, there is provided in circuit 11 an impedance element 18,preferably in the form of a small inductor, and there is also providedan arrangement including a thyratron 19 for applying a cutoff voltage tothe grid of rectifier 14 in response to a voltage developed across theinductor 18. The thyratron circuit includes the plate voltage supplysource 20, plate resistor 21, and switch 22. Voltage source 23establishes the firing level of the thyratron. The R-C combination 24,25 determines the firing time.

In carrying out the test of a transistor, the base of the testtransistor is preferably established at a low reverse potential as bythe source repesrented at 26. The operation of the system to test atransistor is as follows. With the autotransformer 12 initially at itszero position, the operator gradually adjusts it to increase the appliedvoltage and thus cause increase of the collector current. At the sametime, the operator observes the oscilloscope until the current isautomatically cut oif. This occurs when the voltage across inductor 18is sufiicient to cause the thyratron 19 to fire, and the negativevoltage at the plate of the thyratron is applied to the grid ofrectifier 14 to cause abrupt termination of the collector current.

The operator then returns the autotransformer 12 to its zero positionand resets the thyratron by opening switch 22. If desired, thisresetting function may be performed automatically when theautotransforrner is returned to Zero position, as indicated by thebroken line interconnection between the autotransformer and switch 22.Thus the switch may be spring biased to closed position and may be heldopen by the manual control when the latter is in zero position.

If desired, a neon lamp 27 and a current limiting resistor 28 may beprovided as shown, the lamp serving as a visual indicator mainly toindicate resetting of the thyratron, This is particularly desirablewhere there is no interconnection between switch 22 and autotransformer12, and an additional manual operation is necessary, i.e. the momentaryopening of switch 22. The visual indi- 3 cator arrangement helps toinsure that the thyratron is reset before the testing of the nexttransistor is started.

The system provided by this invention, as typified by the embodiment ofFIG. 1, is adaptable for various modes of testing transistors, as willnow be described with the aid 5 of FIG. 2. That figure shows whathappens when a transistor under test is permitted to go into I As thecollector voltage V, is increased from zero, the collector current Iincreases at a substantially constant low rate until the transistor goesinto I whereupon the collector current increases abruptly and continuesto so increase after an abrupt decrease of the collector voltage.Initially, when the collector current increases at a substantiallyconstant low rate, the voltage across inductor 18 is due solely to theIR drop which increases in proportion to the current increase. However,when the transistor goes into I and the collector current increasesabruptly, the voltage across inductor 18 also rises abruptly mainly dueto the inductive effect of the inductor.

As hereinbef-ore stated, the system provided by this in- 20 vention isparticularly useful for determination of the voltage at which each testtransistor goes into I without causing damage to the transistor.Suppose, for example, that the bias voltage at 23 is such that thethyratron will not be fired until the collector current reaches a level25 such as represented at aa in FIG. 2. This will insure that with eachtest transistor the voltage will reach the level at which the transistorgoes into I but the system will cut off the collector current soabruptly that no damage will be done to the transistor. Yet the operatorwill be able to see and note the voltage at which the transistor goesinto I In this way it can be determined whether each test transistor canbe operated with a certain range of applied voltage without going into IIn some instances, it may be desired to observe the I characteristic ofa test transistor by making it go into I In such case, the bias voltageat 23 may be such that the thyratron will not be fired until thecollector current reaches a level such as repsrented at bb in FIG. 2.Then the system will abruptly terminate the collector current.

In some instances, it may be desired to determine whether each testtransistor can be driven to a certain voltage level without going into IFor example, the bias voltage at 23 may be such that the thyratron willfire 5 when the collector current reaches a level such as represented atcc in FIG. 2.

Referring now to FIG. 3, the same system is shown with the addition of apulsing arrangement which reduces the duty cycle of the transistor. Thepulsing arrangement shown, which is merely exemplary, comprises atransistor multivibrator 29 driving a relay control transistor 30 whichin turn operates a mercury relay 31 to intermittently close the circuit11. By way of example, the pulsing operation may be such that the closedand open times are 50 milliseconds and 950 milliseconds, respectively,in which case the duty cycle is only 5%. Such pulsing operation furtherprotects the transistor against damage.

In one physical embodiment of the testing system which 50 is presentlyemployed, the components are as follows:

Voltage source 26 0.5 volt, 10 ohms. 7

While a preferred embodiment of the invention has been illustrated anddescribed, it is to be understood that the invention is not limitedthereto but contemplates such modifications and further embodiments asmay occur to those skilled in the art.

We claim:

1. In a system for testing transistors having an emitter and acollector, a test circuit which in the course of test is connected tothe emitter and collector of a test transistor, means for applying analternating voltage to said circuit, normally-conductive rectifier meansincluded in said circuit so as to produce unidirectional flow ofcollector current therein, said rectifier means comprising at least onevacuum tube including a control grid to which a negative voltage may besupplied to render said tube non-conductive, means controllable by anoperator for gradually increasing the applied alternating voltage toincrease said current, an impedance element in said circuit throughwhich said current flows, means for establishing a predetermined voltagelevel, means responsive only to a voltage developed across said elementin excess of said level for applying a negative cut-off voltage to thecontrol grid of said tube to abruptly terminate said current, and visualindicator means connected to said circuit to indicate desiredinformation to the operator.

2. A system according to claim 1, wherein the current terminating meanscomprises a thyratron triggerable only by a voltage developed acrosssaid impedance element in excess of said level.

3. A system for testing transistors while protecting them against damagein the course of test, comprising a test circuit which in the course oftest has one side connected to the collector of a test transistor andhas its other side connected to the emitter of the test transistor, anelectrical source connected to said circuit to apply voltage thereto andproduce current flow therein through the emitter-collector path of thetest transistor, means manually operable by an operator to increase theapplied voltage and thus increase the current flow in said circuit,visual indicator means connected to said circuit to indicate desiredinformation to the operator, a normallyconductive electronic switchingdevice connected in said circuit, a small impedance element connected insaid circuit in series with said switching device, a thyratron connectedto said impedance element to be fired when the voltage across saidelement reaches a predetermined value, and a connection between saidswitching device and said thyratron for rendering said devicenon-conductive when the thyratron is fired by the voltage across saidelement.

4. A system according to claim 3, further comprising means operable byreturn movement of said manual means for interrupting the operation ofsaid thyratron.

5. A system according to claim 3, further comprising means forintermittently closing said circuit to reduce the duty cycle of the testtransistor.

6. A system for testing transistors While protecting them against damagein the course of test, comprising a test circuit which in the course oftest has one side connected to the collector of a test transistor andhas its other side connected to the emitter of the test transistor, anelectrical source connected to said circuit to apply voltage thereto andproduce current flow therein through the emitter-collector path of thetest transistor, means manually operable by an operator to increase theapplied voltage and thus increase the current flow in said circuit,visual indicator means connected to said circuit to indicate desiredinformation to the operator, a normally-conductive grid-controlledelectron tube connected in said circuit between said source and saidemitter, a small impedance element connected in said circuit betweensaid tube and said emitter, a thyratron having a plate resistorconnected to the end of said impedance element adjacent to said emitter,a connection between the other end of said impedance element and thecontrol grid of said thyratron.

a) whereby said thyratron is controllable by the voltage across saidimpedance element, and a connection between the control grid of saidtube and the plate end of said resistor, whereby said tube is renderednon-conductive when said thyratron is fired by the voltage across saidimpedance element.

References Cited in the file of this patent UNITED STATES PATENTS2,815,446 Coombs Dec. 3, 1957 2,841,746 Mawhinney July 1, 1958 2,845,529Weldon July 29, 1958 6 2,854,615 Light Sept. 30, 1958 2,925,548 SchererFeb. 16, 1960' 2,938,167 Hussey May 24, 1960 OTHER REFERENCES ObtainingTransistor Characteristic Curves, article in Radio and Television News,September 1953 pp. 66, 67, 130 and 131.

Tellerman Electronics, April, 1954, pp. 185-487.

Troubleshooting Data On Selenium Rectifiers, Technic-an, April 1954; pp.34-36 and 63.

Selenium Rectifier Testing, Electronic Technican, February 1957; pp. 32and 33.

